This invention relates to bonding carbon composite friction materials to solid substrates and, in one aspect, to adhesive bonding single ply, woven carbon fabric friction materials to metallic substrates.
Friction transmission and energy adsorption devices, such as clutches, brakes, transmissions and the like, typically include two parts which are normally disengaged and are selectively engaged to cooperatively act as a drive, brake or the like. One or both of the engaging surfaces of these parts usually is a friction material bonded to a metal part. In so-called wet transmission devices, an oil or other suitable cooling medium is force circulated about and between the engaging surfaces of the two parts.
U.S. Pat. No. 4,291,794 Bauer discloses the use of a woven carbon fabric material as the friction material for such applications. The material preferably is singly ply and consists of porous, woven carbon cloth in which individual fibers are coated with pyrolytic carbon. This patent discloses that such a material can be bonded to a metallic backing member by commercially available, high temperature adhesives. Synthetic rubber-phenolic resin base thermosetting adhesives are commonly used for this purpose.
Conventional bonding techniques used with such adhesives typically involves roughening the surface of the metal part by sandblasting, chemical etching, etc., cleaning the surfaces of the friction material and the metal part, applying a thin film of the adhesive in liquid form over the surface one or both of the parts, clamping the parts together after the adhesive has dried and then heating in an oven to cur the adhesive.
It had been found that use of commercially available synthetic rubber-phenolic resin base thermosetting adhesives and such a conventional bonding technique on woven carbon fabric friction materials does not produce a reliable bond. Even though the friction material is quite porous, and therefore an ideal candidate for superior bonding with an adhesive, the shear strength between the adhesive and the friction material, as well as the peel strength, is unacceptable for many applications. The failures almost always are between the friction material and the adhesive, while the bond between the adhesive and the metal part is acceptable.
Even though the amount of adhesive applied to the friction material is carefully controlled, there is a tendency for some to "bleed" through the pores in the friction material and migrate to the outer friction surface when the adhesive passes through the liquid phase during curing. The presence of adhesive on the outer friction surface can adversely affect its friction characteristics. When used in wet transmission devices, the pores of the friction material serve as reservoirs for the cooling oil. If a number of these pores are filled or partially filled with adhesive, there may be insufficient oil to provide the desired coolng.